An emulsion to be applied to the direct positive-type silver halide light-sensitive photographic material needs to be capable of forming good quality images having a sufficiently high maximum density and a sufficiently low minimum density, and also needs to have a sufficiently wide fogging latitude in the light-fogging or chemically fogging process. And in addition, there are fields which require the emulsion to have a soft gradation and a wide exposure latitude.
As the method for widening the fogging latitude by increasing the maximum density and lowering the minimum density, there have been conventionally known the minimum density, there have been conventionally known those methods for its improvement by using antifoggants, development restrainers or selectively spectrally sensitizing dyes. Such methods, however, are disadvantageous in respect that they are unable to give any sufficiently low minimum density, and if the development restrainer is used in a large amount in trying to lower the minimum density, the maximum density (image density) becomes lowered.
As the method for obtaining a wide exposure latitude and a soft gradation by lowering the gamma value, there have been known those methods which use a mixture of direct positive silver halide emulsions different in the sensitivity, for example, a mixture of such emulsions whose sensitivities are varied by using desensitizers. However, they are undesirable in respect of the utilization efficiency of silver and deterioration in the graininess of silver images. For obtaining a softer gradation, the use of a mixture of silver halide emulsions which largely differ in the grain size is also known. However, direct positive silver halide emulsions different in the sensitivity, since their respective fogging latitutdes are different, have the disadvantage that the fogging latitude in the mixture system of the emulsions (overlapped part of their respective latitudes) becomes narrower, so that the resulting image density obtained by development is liable to be fluctuated.
Where silver halides largely different in the grain size exist together, when developed, there occurs a large difference in the developing speed between the larger grain size-having silver halide and the smaller grain size-having silver halide, thus resulting in making them unable to be developed stably against fluctuations in developing conditions.
Further, if the proportion in the grain size (the proportion of the average grain diameter of the smallest average grain size-having silver halide emulsion to the average grain diameter of the largest average grain size-having silver halide emulsion in a light-sensitive material) is excessively large, difference in the developing speed between the respective silver halide emulsions occurs, thus causing the light-sensitive material to be unstable in development, while on the contrary if the proportion in the grain size is to small, the fogging latitude becomes wider to improve the processing stability, but the obtaining of a soft gradation cannot be expected.
In a silver halide light-sensitive photographic material containing at least three layers--a blue-sensitive layer, a green-sensitive layer and a red-sensitive layer, it is essential for the three layers to be well balanced; it is undesirable that even any only one of the layers have a narrow fogging latitude or be unstable in development.